The present invention relates to osteosynthesis devices for the spinal column, the devices comprising a plate and a mechanism for locking a bone screw or anchoring member in position. This divisional application relates to tools used to remove a bone screw after it has been locked in the plate.
U.S. Pat. No. 5,876,402 relates to an osteosynthesis plate comprising through-holes of conical shape capable of housing a bone screw with a completely spherical head to which is clipped, so as to form a ball-joint connection. A split coupling element of conical exterior shape complementing that of the hole is provided. A circlip reduces the aperture of the through-hole. A similar clip and groove arrangement is shown in U.S. Pat. Nos. 5,879,389 and 6,102,952.
In U.S. Pat. No. 5,876,402, the bone screw is placed in the coupling element prior to insertion in the plate. Upon insertion, the split coupling element opens up the circlip. This circlip closes up again once the coupling element has passed through. The coupling element is thus held captive in the through-hole. Final clamping of the anchoring member in position is achieved by the frictional wedging of the coupling element in the bottom of the cone.
In such a system, the number of parts makes the clamping-in position of the anchoring members weak. In addition, the clamping does not occur at the instant when the circlip closes up again after the passage of the coupling element. This leads to the risk of the assembly becoming unclamped, which is prejudicial to the patient.
FIGS. 5 and 6 of U.S. Pat. Nos. 5,879,389 and 6,102,952 show a split-ring for installation in a groove after the bone screw or anchor has been installed in the bone.